It would appear that you can only "forget" SSIDs (wifi networks) that you are attempting to connect to. So attempt to connect to it, select it, then Forget (I think, haven't actually tried it). Actually in my experience with the iPhone, Forget just clears a faulty wifi connection you were attempting to connect to, in order that it can attempt to setup the connection again from the beginning. So typically these SSIDs will just come back. It really doesn't matter as you are only connected to the one you select to connect to.